The Tamoxifen molecule can be seen at the first link below this answer.
The electronegativities we're using are calculated using the Pauling method and can be found in the second link. To judge polarity, we'll use the scale in the third link.
We'll start off by considering the types of bonds that there are in the moledule. We see plenty of Carbon-Carbon bonds, many Carbon-Hydrogen bonds, three Nitrogen-Carbon bonds, and 2 Oxygen-Carbon bonds.
Let's consider the first two, Carbon-Carbon and Carbon-Hydrogen. Recall that the higher the difference in electronegativity, the more polar a bond is. Clearly, Carbon-Carbon bonds in this molecule will be nonpolar because there is no difference in electronegativity. The electronegativity difference between Carbon and Hydrogen is below:
EN(Carbon) - EN(Hydrogen) = 2.55 - 2.20 = 0.35
0.35 is right on the line (0.30 is roughly the line for polar vs. nonpolar), and traditionally, C-H bonds are considered nonpolar, so we'll leave these alone.
Now, let's consider the Carbon-Nitrogen bonds.
EN(Nitrogen) - EN(Carbon) = 3.04 - 2.55 = 0.49
This bond is clearly polar! This is good, because there are 3 Carbon-Nitrogen bonds in the molecule, so we found 3 out of 5 of our polar bonds.
Finally, let's look at the Carbon-Oxygen bonds in the ether group.
EN(Oxygen) - EN(Carbon) = 3.44 - 2.55 = 0.89
Again, this is clearly polar! On top of that, this is more polar than the Carbon-Nitrogen bonds!
So, we have the 5 polar bonds in the Tamoxifen molecule: 3 Carbon-Nitrogen bonds and 2 Carbon-Oxygen bonds.
To explain the polarity of these bonds, we just need to look at electronegativity and what it means. Electronegativity is the tendency of an atom to attract electrons to itself. One of the trends of the periodic table is to have electronegativity increase as we move to the right, which makes sense! As we move to the right (especially in the nonmetals), atoms in their elemental states have a greater difference in their energy as elements and the lower energy when they have a full valence shell. As this difference widens, the atom "wants" more and more to grab electrons to get to its lowest-energy state. This is why Oxygen and Nitrogen are so much more hungry for electrons than Carbon is.
You need to look at the chemical structure and chemical formula of tamoxifen: C_26H_29NO.
You need to eliminate the bonds that are not polar, hence C-H and C-C bonds are not polar, hence, they are not considered.
Since O and N are more electronegative atoms, hence, they will unequally share electrons with C atoms, developing polar covalent bonds.
The chemical structure shows that three bonds link C atoms to N atoms and two bonds that link O atoms to C. The 5 bonds are polar covalent bonds.